1. Field of the Invention
The present invention relates to a stepping motor suitable for driving the photographing lens of a single-lens reflex camera or a video camera and a method of manufacturing the motor.
2. Related Background Art
Conventionally, a two-phase PM type stepping motor as shown in FIG. 1 is used as a stepping motor for driving the photographing lens of a single-lens reflex camera or a video camera. Referring to FIG. 1, a motor shaft 1 is rotatably positioned by an intermediate bearing (normally, a sintered oil-impregnated alloy bearing) 3 and a distal end bearing 4 both of which are pressed into a motor holder 2. A thrust spring 5 suppresses any play of the motor shaft along the thrust direction and applies an appropriate pilot pressure to the motor shaft. A cylindrical magnet rotor 6 is fixed to the motor shaft 1 by bonding, and the outer surface of the magnet rotor 6 is magnetized to have multiple poles.
A stator 7 comprises two electromagnets each of which has pole teeth corresponding to the number of poles of the magnet rotor and is constituted by coils 11 as solenoids wound on a bobbin 10 to form a magnetic path together with a motor case 9 which is coupled to the motor holder with screws 8. The pole teeth of the electromagnets are shifted by a 1/2 pitch. More specifically, when rectangular waves or sine waves that are 90.degree. out of phase are supplied to the coils, the magnet rotor 6 rotates. A nut or rack (not shown) meshed with a screw portion 1a of the motor shaft 1 can be used to move a lens.
In this conventional stepping motor, when the motor shaft 1 is to be inserted in the intermediate bearing 3 of the holder 2, the screw portion 1a of the motor shaft 1 may be damaged, and the resultant motor characteristics may be degraded. In addition, a post-process is required to match the screw diameter of the screw portion 1a of each of the mass-produced motor shafts 1 with the diameter of the intermediate bearing 3. Furthermore, a large number of parts are used to result in an increase in cost.